Vehicles may be fitted with evaporative emission control systems (EVAP) to reduce the release of fuel vapors to the atmosphere. For example, a fuel vapor canister packed with an adsorbent may adsorb and store refueling, running loss, and diurnal fuel vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel.
Since leaks in the emissions control system can inadvertently allow fuel vapors to escape to the atmosphere, leak detection routines may be intermittently performed. Therein, a negative pressure is applied on the fuel system (such as engine manifold vacuum or negative pressure from an alternate vacuum source) until a target vacuum is reached, following which the fuel system is sealed and a rate of pressure decay is monitored. Based on the rate of pressure bleed-up, fuel system leaks may be identified.
Based on the ideal gas law, a sudden change in fuel tank pressure can be attributed to various factors, such as quantity of fuel in the fuel tank, fuel volume, and fuel temperature. While the fuel tank is designed to withstand the negative pressure applied during a leak test, as well as some pressure fluctuations, there may be conditions when there is more vacuum in the fuel tank than commanded. As an example, if there is a leaky canister purge valve, more vacuum may be applied to the fuel tank than desired. If the fuel tank has excessive vacuum, fuel tank oil-canning may occur wherein the fuel tank volume may suddenly change and tank integrity may be compromised. The fuel tank may flex and damage one or more sensors and actuators coupled to the tank, such as a fuel level indicator and a fuel pump. Rapid and accurate identification of oil-canning may be required to take countermeasures to reduce fuel tank damage.
The inventors herein have recognized that during oil-canning events, there may be a momentary, and drastic, change in fuel tank volume and pressure. By ruling out other factors that affect fuel tank pressure, a sudden pressure rise may be correlated with an oil-canning event, and appropriate mitigating steps may be taken in a timely fashion. Thus in one example, fuel tank oil-canning may be accurately diagnosed by a method comprising: indicating fuel tank oil-canning in response to a higher than threshold rise in fuel tank pressure during or following application of vacuum to the fuel tank while each of a fuel temperature and fill level remain within a range.
As an example, a vacuum (e.g., an intake manifold vacuum) may be applied to a fuel tank when leak test conditions are met, or when canister purging conditions are met. If a sudden drop in fuel tank vacuum and/or a sudden drastic rise in fuel tank pressure is seen following application of vacuum to the fuel tank, a controller may perform a diagnostic routine to determine if the sudden change in fuel tank pressure is due to oil-canning at the fuel tank. Specifically, oil-canning may be inferred if the change in fuel tank pressure cannot be attributed to any change in fuel tank temperature (e.g., due to the tank temperature staying substantially constant) or change in fuel vapor amount (e.g., due to no fuel sloshing events) incurred during the vacuum application. If oil-canning is inferred, the application of vacuum may be discontinued and the fuel tank may be vented to the atmosphere to relieve pressure and protect the fuel tank. In this way, oil-canning may be detected rapidly and accurately. By improving the accuracy of oil-canning detection, appropriate mitigating steps may be taken to reduce the unintended degradation of the fuel tank and one or more sensors and actuators coupled to it. Overall, fuel system integrity can be better maintained.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.